Control of wet scrubber oxidation inhibitor and byproduct recovery

ABSTRACT

The present disclose is directed to a method for controlling iodine levels in wet scrubbers, and, in particular, recirculating wet scrubbers by removing the iodine from the scrubbing solution, such as by using ion exchange, absorption, adsorption, precipitation, filtration, solvent extraction, ion pair extraction, and an aqueous two-phase extraction.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent application Ser. No. 16/188,758 filed Nov. 13, 2018 which is a continuation of U.S. patent application Ser. No. 15/694,536, filed Sep. 1, 2017, now U.S. Pat. No. 10,159,931, with an issue date of Dec. 25, 2018, which is a continuation of U.S. patent application Ser. No. 15/217,749, filed Jul. 22, 2016, now U.S. Pat. No. 9,889,405, with an issue date of Feb. 13, 2018, which is a continuation of U.S. patent application Ser. No. 14/512,142, filed Oct. 10, 2014, now U.S. Pat. No. 9,409,123, with an issued date of Aug. 9, 2016, which is a continuation of U.S. patent application Ser. No. 13/861,162, filed Apr. 11, 2013, now U.S. Pat. No. 8,883,099 with an issued date of Nov. 11, 2014, which claims the benefits of U.S. Provisional Application Ser. No. 61/622,728, filed Apr. 11, 2012, all having the same title, each of which is incorporated herein by this reference in its entirety.

FIELD

The disclosure relates generally to mercury removal from waste gas and particularly to halogen-assisted mercury removal from waste gas.

BACKGROUND

Plants that have a majority of elemental mercury emissions in the discharge flue gas (or waste gas) may utilize halogen additives to control mercury emissions. One primary effect of a halogen additive is to promote oxidized mercury species (Hg⁺⁺) in the flue gas. Mercury emissions are generally reduced by two mechanisms: (a) adsorption of mercury (Hg⁺⁺ in particular) on particles or particulates in the flue gas and subsequent removal by the particulate control device; and/or (b) absorption of Hg⁺⁺ in a flue-gas desulfurization (“FGD”) scrubber. Among halogens, bromine and bromide additives and brominated sorbents are widely employed for mercury control from coal-fired sources. However, there are serious emerging problems associated with the use of high concentration bromide additives and sorbents. Applied as coal additives, iodine and iodide salts are an alternative to bromine.

There is a need to optimize performance and reduce operating costs of iodine additives for mercury control for plants with wet FGD scrubbers. Iodine or iodide compounds are often added onto the coal feed at a rate of about 1 to 30 ppm_(w) of coal feed. Iodine is a relatively expensive compared to bromine, but is about 10 times more efficient as a mercury oxidizer. Compared to bromine and bromide additives, iodine and iodide salts can have fewer detrimental side effects, such as metal corrosion, and has less potential to create emissions of stratospheric ozone-destroying precursors (due in part to its use at much lower concentrations).

There is also a need to control iodine buildup in flue gas treatment processes, even where iodine and/or iodide salts are not added. Many coals used in utility plants have higher native iodine concentrations (>3 ppm_(w)). The majority of North American coals and lignites (for which there is measured iodine data) have low iodine concentration (<1 ppm_(w)). However, iodine is not comprehensively monitored in North American coals. Iodine is known to accumulate in certain coal formations such as marine roof coals and in select coals associated with volcanic activity.

Native iodine in the coal and/or iodine added to the coal feed is believed to first form hydroiodic acid (HI) as it cools after combustion. Further reactions convert a portion of this to molecular I₂ gas. Iodide in flue gas as hydroiodic acid is water soluble and will be retained in the scrubber slurry. Iodine is less soluble in solution, but solubility is increased in a mixed iodide/iodine solution. In solution, the molecular iodine reacts reversibly with the negative I⁻ ion, generating the I₃ ⁻ anion, which is soluble in water.

An excess of iodine dissolved in scrubber solution can not only be volatilized during upset conditions, leading to a characteristic “purple plume” stack emission but also interfere with acid gas removal. Iodine is an oxidation inhibitor. High concentrations of iodide/iodine in the scrubbing solution moderate the sulfite oxidation rate and suppress the oxidation reduction potential (ORP). Iodide is able to reduce either the sulfite or the peroxomonosulfate radicals or the catalytically active transition-metal ions and is thereby oxidized to iodine. It can subsequently be reduced to iodide again by excess sulfite. Therefore, iodine is able to inhibit the overall SO₂ to sulfating reaction(s) and is not consumed in the process.

SUMMARY

These and other needs are addressed by the various aspects, embodiments, and configurations of the present disclosure. The present disclose is directed generally to controlling iodine and/or iodide levels in web scrubbers.

A method, according to the present disclosure, can include the steps of:

(a) receiving a waste gas comprising an acid gas and elemental iodine and/or an iodine-containing composition;

(b) passing the waste gas through a wet scrubber to remove, from the waste gas, at least a portion of the acid gas and the elemental iodine and/or iodine-containing composition and form a scrubbing solution comprising an acid derived from the acid gas and/or a derivative thereof and the elemental iodine and/or iodine-containing composition; and

(c) removing, from the scrubbing solution, at least a portion of the elemental iodine and/or iodine-containing composition, thereby reducing a concentration of the elemental iodine and/or iodine-containing composition in the scrubbing solution.

Another method, according to this disclosure, can include the steps of:

(a) receiving a waste gas comprising an acid gas and elemental mercury;

(b) contacting an iodine-containing additive with the waste gas upstream of a wet scrubber to oxidize elemental mercury in the waste gas to a non-elemental mercury

(c) passing the waste gas through a wet scrubber to remove at least a portion of the acid gas and elemental iodine/or an iodine-containing composition derived from the iodine-containing additive and form a scrubbing solution comprising an acid and/or salt derived from the acid gas and the elemental iodine and iodine-containing composition; and

(d) removing, from the scrubbing solution, at least a portion of the elemental iodine and/or iodine-containing composition, thereby reducing a concentration of the elemental iodine and/or iodine-containing composition in the scrubbing solution.

Another method, according to the disclosure, can include the steps of:

(a) receiving a waste gas comprising elemental iodine and/or an iodine-containing composition;

(b) passing the waste gas through a wet scrubber to remove the elemental iodine and/or iodine-containing composition from the waste gas and form a scrubbing solution comprising the elemental iodine and/or an iodine-containing composition; and

(c) removing, from the scrubbing solution, at least a portion of the elemental iodine and/or iodine-containing composition by one or more of ion exchange, absorption, adsorption, precipitation, filtration, solvent extraction, ion pair extraction, and aqueous two-phase extraction, thereby reducing a concentration of the elemental iodine and/or iodine-containing composition in the scrubbing solution.

When the waste gas includes elemental mercury, the method can include the step of contacting an iodine-containing additive with the waste gas upstream of the wet scrubber to oxidize elemental mercury in the waste gas to a non-elemental mercury.

The elemental iodine and/or iodine-containing composition can be removed from the scrubbing solution by a carbonaceous material. The elemental iodine and/or iodine-containing composition can be sorbed on the carbonaceous material. The method can further include the steps of:

recovering the carbonaceous material from the scrubbing solution; and

introducing the recovered carbonaceous material to the waste gas as the iodine-containing additive.

Iodine or iodide compounds are commonly added onto the coal feed at a rate of about 1 to 30 ppm_(w) of coal feed.

In some applications, the method can include the further steps of:

contacting an iodine-containing additive with a mercury-containing feed material, the waste gas being derived from the mercury-containing feed material;

recovering the carbonaceous material from the scrubbing solution; and

introducing the recovered carbonaceous material to the waste gas as the iodine-containing additive.

The removing step can include passing at least a portion of the scrubbing solution through a carbonaceous material to collect onto the carbonaceous material the elemental iodine and/or iodine-containing composition.

After the removing step, the scrubbing solution can be returned to the wet scrubber, particularly when the wet scrubber is a flue-gas desulfurization scrubber.

In some applications, the flow of the scrubbing solution through the carbonaceous material is controlled at a rate to maintain overall iodine concentration in the scrubber solution at less than about 100 ppm.

In some applications, the flow of an input scrubbing solution through a first bed of the carbonaceous material is stopped automatically by a microprocessor when a concentration of iodine and/or iodine-containing composition in an output scrubbing solution is at least a predetermined threshold and/or a difference between concentrations of the at least one of iodine and iodine-containing composition in the input and output scrubbing solutions is at least a predetermined threshold. The flow of the scrubbing solution through a second bed of the carbonaceous material can be initiated automatically by the microprocessor in response to an absolute value of the concentration of the iodine and iodine-containing composition in the output scrubbing solution reaching the at least a predetermined threshold.

The carbonaceous material can be introduced into the waste gas at an inlet to the wet scrubber or into a reservoir of the scrubbing solution in the wet scrubber. For example, activated carbon or carbonaceous sorbent can be introduced as a powdered sorbent upstream of the wet FGD scrubber or into the recycle loop or in the form of a granular packed bed treatment in the recycle loop.

The scrubbing solution can be contacted with an oxidizing agent and/or pH-adjusted to convert elemental mercury in the scrubbing solution and/or on the carbonaceous material to a more water soluble form of mercury and convert iodide to elemental iodine for collection by the carbonaceous material.

By way of example, iodine concentration in flue-gas desulfurization (“FGD”) scrubbing (slurry) solutions can be controlled by selective sorption of iodine (I₂) on suspended carbonaceous sorbents, such as activated carbon. Soluble iodide (I⁻) in the slurry may be oxidized to iodine to remove a portion of the iodine during normal scrubber operations including forced oxidation or via a separate oxidation step of the scrubber wastewater treatment stream.

To enhance elemental mercury oxidation, the method can include one or more of the following steps upstream of the wet scrubber:

(i) contacting the waste gas with at least one of a halogen and a halogen-containing composition;

(ii) contacting the waste gas with a selective catalytic reduction catalyst; and

(iii) contacting the waste gas with a carbonaceous material in a baghouse.

In the third option, the carbonaceous material can be introduced into the waste gas at or near an inlet of the baghouse. Mercury oxidation rate at the FGD inlet may be increased by flue gas transit through fabric filter baghouse with select fabrics, ash cake characteristics, and sorbent and catalytic oxidizers optionally added.

The various methods can apply, control, recycle, and/or recover iodine to enhance mercury recovery and maintain SO₂ removal efficiency, particularly in wet flue gas desulfurization (FGD) scrubbers on coal-fired power plants. Microporous carbonaceous sorbents, such as activated carbon, can be excellent selective sorbents for elemental iodine (but typically not of iodide). Sorbed iodine on the carbon surface can increase chemisorption of mercury on the carbon surface, increase overall removal of mercury and prevent re-emission of mercury from the scrubber. Controlling buildup of iodide and iodine species in scrubber solution in wet FGD scrubbers can maintain oxidation efficiency of wet FGD scrubbers. Carbonaceous materials enriched in iodine can be separated from the scrubbing slurry effectively, further treated to remove mercury and recharged to the coal feed. Alternatively the recovered carbonaceous material can be landfilled and/or the iodine may be recovered for commercial sale, particularly for plants that fire high iodide coals or that use iodine coal additives for mercury control.

The present disclosure can provide a number of other advantages depending on the particular configuration. This disclosure can provide a means to supplementally control iodide buildup and, optionally, to reduce the amount of halogen coal additive required by recycling a portion of the slurry iodide back to the coal feed. It can substantially optimize performance and reduce operating costs of iodine additives for mercury control for plants with wet FGD scrubbers. Although iodine is relatively expensive compared to bromine, it is about 10 times more efficient than other halogens, particularly bromine, as a mercury oxidizer. It can have fewer detrimental side effects such as metal corrosion and less potential to create emissions of stratospheric ozone-destroying precursors (due in part to iodine's use at much lower concentrations). The disclosure can also be useful for plants that fire coals or other bio-mass with higher native iodine concentrations (>3 ppm_(w)). The majority of North American coals and lignites for which there is measured iodine data have low iodine concentration (<1 ppm_(w)). Iodine is known to accumulate in certain coal formations such as marine roof coals and in select coals associated with volcanic activity. For plants with a fabric filter followed by a wet scrubber, mercury oxidation across the fabric filter can be enhanced by control of various operating parameters to maximize mercury capture in the scrubber.

“A” or “an” entity refers to one or more of that entity. As such, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, and “having” can be used interchangeably.

“Absorption” is the incorporation of a substance in one state into another of a different state (e.g. liquids being absorbed by a solid or gases being absorbed by a liquid). Absorption is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase—gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface (as in the case for adsorption).

“Adsorption” is the adhesion of atoms, ions, biomolecules, or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid. Similar to surface tension, adsorption is generally a consequence of surface energy. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces)) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.

“Acid gas” refers to any type of gas or gaseous mixture which forms an acidic compound when mixed with water. The most common types of acid gases are hydrogen sulfide (H₂S), sulfur oxides (SO_(X)) (which can form sulfuric acid when mixed with water), nitric oxides (NO_(X)) (which can form nitric acid when mixed with water), and carbon monoxide (CO) and/or carbon dioxide (CO₂) (which can form carbonic acid when mixed with water).

“Ash” refers to the residue remaining after complete combustion of the coal particles. Ash typically includes mineral matter (silica, alumina, iron oxide, etc.).

“At least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. When each one of A, B, and C in the above expressions refers to an element, such as X, Y, and Z, or class of elements, such as X₁-X_(n), Y₁-Y_(m), and Z₁-Z_(o), the phrase is intended to refer to a single element selected from X, Y, and Z, a combination of elements selected from the same class (e.g., X₁ and X₂) as well as a combination of elements selected from two or more classes (e.g., Y₁ and Z_(o)).

“Biomass” refers to biological matter from living or recently living organisms. Examples of biomass include, without limitation, wood, waste, (hydrogen) gas, seaweed, algae, and alcohol fuels. Biomass can be plant matter grown to generate electricity or heat. Biomass also includes, without limitation, plant or animal matter used for production of fibers or chemicals. Biomass further includes, without limitation, biodegradable wastes that can be burnt as fuel but generally excludes organic materials, such as fossil fuels, which have been transformed by geologic processes into substances such as coal or petroleum. Industrial biomass can be grown from numerous types of plants, including miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum, sugarcane, and a variety of tree species, ranging from eucalyptus to oil palm (or palm oil).

“Carbonaceous” refers to a carbon-containing material, particularly a material that is substantially rich in carbon.

“Coal” refers to a combustible material formed from prehistoric plant life. Coal includes, without limitation, peat, lignite, sub-bituminous coal, bituminous coal, steam coal, anthracite, and graphite. Chemically, coal is a macromolecular network comprised of groups of polynuclear aromatic rings, to which are attached subordinate rings connected by oxygen, sulfur, and aliphatic bridges.

A “composition” refers to one or more chemical units composed of one or more atoms, such as a molecule, polyatomic ion, chemical compound, coordination complex, coordination compound, and the like. As will be appreciated, a composition can be held together by various types of bonds and/or forces, such as covalent bonds, metallic bonds, coordination bonds, ionic bonds, hydrogen bonds, electrostatic forces (e.g., van der Waal's forces and London's forces), and the like.

“Flue-gas desulfurization” or “FGD” refers to a set of technologies to remove gas-phase sulfur dioxide (SO₂), particularly from exhaust flue gases of fossil-fuel power plants and from the emissions of other sulfur oxide emitting processes.

“Halogen” refers to an electronegative element of group VIIA of the periodic table (e.g., fluorine, chlorine, bromine, iodine, astatine, listed in order of their activity with fluorine being the most active of all chemical elements).

“Halide” refers to a binary compound of the halogens.

“High alkali coals” refer to coals having a total alkali (e.g., calcium) content of at least about 20 wt. % (dry basis of the ash), typically expressed as CaO, while “low alkali coals” refer to coals having a total alkali content of less than 20 wt. % and more typically less than about 15 wt. % alkali (dry basis of the ash), typically expressed as CaO.

“High iron coals” refer to coals having a total iron content of at least about 10 wt. % (dry basis of the ash), typically expressed as Fe₂O₃, while “low iron coals” refer to coals having a total iron content of less than about 10 wt. % (dry basis of the ash), typically expressed as Fe₂O₃. As will be appreciated, iron and sulfur are typically present in coal in the form of ferrous or ferric carbonates and/or sulfides, such as iron pyrite.

“High sulfur coals” refer to coals having a total sulfur content of at least about 1.5 wt. % (dry basis of the coal) while “medium sulfur coals” refer to coals having between about 1.5 and 3 wt. % (dry basis of the coal) and “low sulfur coals” refer to coals typically having a total sulfur content of less than about 1.5 wt. % (dry basis of the coal), more typically having a total sulfur content of less than about 1.0 wt. %, and even more typically having a total sulfur content of less than about 0.8 wt. % of the coal (dry basis of the coal).

“Iodine number” is used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation), often reported in mg/g (typical range 500-1200 mg/g). It is a measure of the micropore content of the activated carbon (0 to 20 Å, or up to 2 nm) by adsorption of iodine from solution. It is equivalent to surface area of carbon between 900 m²/g and 1100 m²/g. Iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the residual filtrate is 0.02 normal. Basically, iodine number is a measure of the iodine adsorbed in the pores and, as such, is an indication of the pore volume available in the activated carbon of interest.

“Ion exchange medium” refers to a medium that is able, under selected operating conditions, to exchange ions between two electrolytes or between an electrolyte solution and a complex. Examples of ion exchange resins include solid polymeric or mineralic “ion exchangers”. Other exemplary ion exchangers include ion exchange resins (functionalized porous or gel polymers), zeolites, montmorillonite clay, clay, and soil humus. Ion exchangers are commonly either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are: H⁺ (proton) and OW (hydroxide); single-charged monoatomic ions like Na⁺, K⁺, and Cl⁻; double-charged monoatomic ions like Ca²⁺ and Mg²⁺; polyatomic inorganic ions like SO₄ ²⁻ and PO₄ ³⁻; organic bases, usually molecules containing the amino functional group —NR₂H⁺; organic acids often molecules containing —COO⁻ (carboxylic acid) functional groups; and biomolecules that can be ionized: amino acids, peptides, proteins, etc.

The term “means” as used herein shall be given its broadest possible interpretation in accordance with 35 U.S.C., Section 112, Paragraph 6. Accordingly, a claim incorporating the term “means” shall cover all structures, materials, or acts set forth herein, and all of the equivalents thereof. Further, the structures, materials or acts and the equivalents thereof shall include all those described in the summary of the invention, brief description of the drawings, detailed description, abstract, and claims themselves.

Neutron Activation Analysis (“NAA”) refers to a method for determining the elemental content of samples by irradiating the sample with neutrons, which create radioactive forms of the elements in the sample. Quantitative determination is achieved by observing the gamma rays emitted from these isotopes.

“Oxidizing agent”, “oxidant” or “oxidizer” refers to an element or compound that accepts one or more electrons to another species or agent that is oxidized. In the oxidizing process the oxidizing agent is reduced and the other species which accepts the one or more electrons is oxidized. More specifically, the oxidizer is an electron acceptor, or recipient, and the reductant is an electron donor or giver.

“Particulate” refers to fine particles, such as fly ash, unburned carbon, soot and fine process solids, typically entrained in a gas stream.

“pH Adjustor” refers to any material, whether acidic, basic, or alkaline, that adjusts the pH of a solution. Exemplary basic or alkaline materials include alkali and alkaline earth metal hydroxides, carbonates, and ammonia and acidic materials include sulfuric acid, hydrochloric acid, nitric acid, carbonic acid, phosphoric acid, and other mineral acids.

The phrase “ppmw X” refers to the parts-per-million, based on weight, of X alone. It does not include other substances bonded to X.

The phrase “ppmv X” refers to the parts-per-million, based on volume, of X alone. It does not include other substances bonded to X.

“Reducing agent”, “reductant” or “reducer” refers to an element or compound that donates one or more electrons to another species or agent this is reduced. In the reducing process the reducing agent is oxidized and the other species which accepts the one or more electrons is oxidized. More specifically, the reducer is an electron donor and the oxidant is an electron acceptor or recipient.

The terms “remove” or “removing” include the sorption, precipitation, adsorption, absorption, conversion, deactivation, decomposition, degradation, neutralization, and/or killing of a target material.

A “scrubber” or “scrubber system” is an air pollution control device that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term “scrubber” has referred to a pollution control device to “wash out” acid gases in an exhaust stream, such as a flue gas.

“Separating” and cognates thereof refer to setting apart, keeping apart, sorting, removing from a mixture or combination, or isolating. In the context of gas mixtures, separating can be done by many techniques, including electrostatic precipitators, baghouses, scrubbers, and heat exchange surfaces.

“Soluble” refers to materials that readily dissolve in water. For purposes of this invention, it is anticipated that the dissolution of a soluble compound would necessarily occur on a time scale of minutes rather than days. For the compound to be considered to be soluble, it is necessary that it has a significantly high solubility product such that upwards of 5 g/L of the compound will be stable in solution.

A “sorbent” is a material that sorbs another substance; that is, the material has the capacity or tendency to take it up by sorption.

“Sorb” and cognates thereof mean to take up a liquid or a gas by sorption.

“Sorption” and cognates thereof refer to adsorption and absorption, while desorption is the reverse of adsorption.

Unless otherwise noted, all component or composition levels are in reference to the active portion of that component or composition and are exclusive of impurities, for example, residual solvents or by-products, which may be present in commercially available sources of such components or compositions.

All percentages and ratios are calculated by total composition weight, unless indicated otherwise.

It should be understood that every maximum numerical limitation given throughout this disclosure is deemed to include each and every lower numerical limitation as an alternative, as if such lower numerical limitations were expressly written herein. Every minimum numerical limitation given throughout this disclosure is deemed to include each and every higher numerical limitation as an alternative, as if such higher numerical limitations were expressly written herein. Every numerical range given throughout this disclosure is deemed to include each and every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written herein.

The preceding is a simplified summary of the disclosure to provide an understanding of some aspects of the disclosure. This summary is neither an extensive nor exhaustive overview of the disclosure and its various aspects, embodiments, and configurations. It is intended neither to identify key or critical elements of the disclosure nor to delineate the scope of the disclosure but to present selected concepts of the disclosure in a simplified form as an introduction to the more detailed description presented below. As will be appreciated, other aspects, embodiments, and configurations of the disclosure are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of the specification to illustrate several examples of the present disclosure. These drawings, together with the description, explain the principles of the disclosure. The drawings simply illustrate preferred and alternative examples of how the disclosure can be made and used and are not to be construed as limiting the disclosure to only the illustrated and described examples. Further features and advantages will become apparent from the following, more detailed, description of the various aspects, embodiments, and configurations of the disclosure, as illustrated by the drawings referenced below.

FIG. 1 is a block diagram according to an embodiment;

FIG. 2 is a block diagram according to an embodiment;

FIG. 3 is a block diagram according to an embodiment;

FIG. 4 is a block diagram according to an embodiment;

FIG. 5 is a block diagram according to an embodiment; and

FIG. 6 is a block diagram according to an embodiment.

DETAILED DESCRIPTION Process Overview

The current disclosure is directed to the use of an iodine-containing additive, typically present in relatively low concentrations, and/or natively occurring iodine-containing compounds in feed materials, such as coal, to control mercury emissions from vapor phase mercury evolving facilities, such as smelters, autoclaves, roasters, steel foundries, steel mills, cement kilns, power plants, waste incinerators, boilers, and other mercury-contaminated gas stream producing industrial facilities. Although the mercury is typically evolved by combustion, it may be evolved by other oxidation and/or reducing reactions, such as roasting, autoclaving, and other thermal processes that expose mercury containing materials to elevated temperatures.

The current disclosure is further directed to the application, control, recycle and recovery of iodine to enhance mercury recovery and to maintain SO₂ removal efficiency in wet flue gas desulfurization (FGD) scrubbers. Iodide concentration in slurry solution is controlled by oxidation of iodide to iodine in scrubber waste water followed by selective sorption of the generated iodine on suspended carbonaceous sorbents, such as activated carbon or by iodide ion (anion) exchange.

Iodine enriched sorbent may be separated from solution, dewatered and recharged onto the coal feed for combustion if concentration of heavy metals such as mercury and selenium on the sorbent are kept at low levels. A portion of the iodine required for coal treatment can thereby be provided by recycle. Alternatively, iodine can be extracted from the activated carbon for further purification and separation for commercial use.

While not wishing to be bound by any theory, native iodine in the coal or iodine added to the coal feed is believed to first form hydroiodic acid (HI) as it cools after combustion. Further reactions are believed to convert a portion of this to molecular I₂ gas. Iodide in flue gas as hydroiodic acid is water soluble and will be retained in the scrubber slurry. Iodine can be less soluble in solution, but solubility can be increased in a mixed iodide/iodine solution. In solution, the molecular iodine reacts reversibly with the negative I⁻ ion, generating the I₃ ⁻ anion, which is soluble in water. An excess of iodine dissolved in scrubber solution can be volatilized during upset conditions, leading to a characteristic “purple plume” stack emission. Maintaining lower iodine and iodide levels in the scrubbing solution can reduce or eliminate entirely such undesirable emissions.

FIG. 1 depicts a common contaminated gas stream treatment process for an industrial facility. Referring to FIG. 1, a mercury-containing feed material 100 is provided. In one application, the feed material 100 is combustible and can be any synthetic or natural, mercury-containing, combustible, and carbon-containing material, including coal and biomass. The feed material 100 can be a high alkali or high iron coal. In other applications, the present disclosure is applicable to noncombustible, mercury-containing feed materials, including without limitation metal-containing ores, concentrates, and tailings.

The feed material 100 can natively include, without limitation, varying levels of halogens (e.g., iodine, bromine, and chlorine) and mercury. Typically, the feed material 100 includes typically at least about 0.001 ppmw, even more typically from about 0.003 to about 100 ppmw, and even more typically from about 0.003 to about 10 ppmw mercury (both elemental and speciated) (measured by neutron activation analysis (“NAA”)). Commonly, a combustible feed material 100 includes no more than about 5 ppmw iodine, more commonly no more than about 4 ppmw iodine, even more commonly no more than about 3 ppmw iodine, even more commonly no more than about 2 ppmw iodine and even more commonly no more than about 1 ppmw iodine (measured by neutron activation analysis (“NAA”)). A combustible feed material 100 generally will produce, upon combustion, an unburned carbon (“UBC”) content of from about 0.1 to about 30% by weight and even more generally from about 0.5 to about 20% by weight.

The feed material 100 is combusted in thermal unit 104 to produce a mercury-containing gas stream 108. The thermal unit 104 can be any combusting device, including, without limitation, a dry or wet bottom furnace (e.g., a blast furnace, puddling furnace, reverberatory furnace, Bessemer converter, open hearth furnace, basic oxygen furnace, cyclone furnace, stoker boiler, cupola furnace and other types of furnaces), boiler, incinerator (e.g., moving grate, fixed grate, rotary-kiln, or fluidized or fixed bed, incinerators), calciners including multi-hearth, suspension or fluidized bed roasters, intermittent or continuous kiln (e.g., ceramic kiln, intermittent or continuous wood-drying kiln, anagama kiln, bottle kiln, rotary kiln, catenary arch kiln, Feller kiln, noborigama kiln, or top hat kiln), oven, or other heat generation units and reactors.

The mercury-containing gas stream 108 includes not only elemental and/or speciated mercury but also a variety of other materials. A common mercury-containing gas stream 108 includes at least about 0.001 ppmw, even more commonly at least about 0.003 ppmw, and even more commonly from about 0.005 to about 0.02 ppmw mercury (both elemental and speciated). Other materials in the mercury-containing gas stream 108 can include, without limitation, particulates (such as fly ash), sulfur oxides, nitrogen oxides, carbon oxides, unburned carbon, halogens, and other types of particulates.

The temperature of the mercury-containing gas stream 108 varies depending on the type of thermal unit 104 employed. Commonly, the mercury-containing gas stream 108 temperature is at least about 125° C., even more commonly is at least about 325° C., and even more commonly ranges from about 325 to about 500° C.

The mercury-containing gas stream 108 is optionally passed through an economizer 112 and/or a an air preheater 120 to transfer some of the thermal energy of the mercury-containing gas stream 108 to air input to the thermal unit 104. The heat transfer produces a common temperature drop in the mercury-containing gas stream 108 of from about 50 to about 300° C. to produce a mercury-containing gas stream 116 temperature commonly ranging from about 100 to about 400° C.

The mercury-containing gas stream 108 can also optionally be passed through a selective catalytic reduction (“SCR”) zone 116. As will be appreciated, SCR converts nitrogen oxides, or NO_(X), with the aid of a catalyst, into diatomic nitrogen (N₂) and water. The SCR can also increase the oxidation of Hg at the wet scrubber inlet (discussed below). A gaseous reductant or reducing agent, typically anhydrous ammonia, aqueous ammonia, or urea (but other gas-phase reductants may be employed), can be injected into a stream of flue or exhaust gas or other type of gas stream or absorbed onto a catalyst followed by off gassing of the ammonia into the gas stream. Suitable catalysts include, without limitation, ceramic materials used as a carrier, such as titanium oxide, and active catalytic components, such as oxides of base metals (such as vanadium (V₂O₅), wolfram (WO₃), titanium oxide (TiO₂) and tungstate (e.g., WO₄ ²⁻), zeolites, and various precious metals. Other catalysts, however, may be used.

The SCR catalyst surface, depending on the design, catalyst and layering, is active for reactions other than the primary nitrogen oxide reduction. There are competing reactions occurring for available sites to reduce NO_(X), oxidize SO₂ to SO₃ and to promote the reaction of mercury with various species to result in an increased fraction of oxidized mercury species. The SCR ammonia rate is co-variable with load and temperature and affects the balance between these competing reactions.

The mercury-containing gas stream 108 is next subjected to particulate removal device 124 to remove most of the particulates from the mercury-containing gas stream and form a treated gas stream 128. The particulate removal device 124 can be any suitable device, including an electrostatic precipitator, particulate filter such as a baghouse, wet particulate scrubber, and other types of particulate removal devices. In one configuration a reverse air or pulsed jet baghouse is employed.

The treated gas stream 128 is next passed through a wet scrubber 132, typically a wet FGD scrubber to remove at least most and more typically at least about 75% of any remaining particulates and acid gases in the treated gas stream 408 to form a further treated gas stream 136. An exemplary scrubber is a limestone forced oxidation LSFO scrubber for SO_(X), heavy metal and mercury control. As will be appreciated, SO₂ is an acid gas, and, therefore, the typical sorbent slurries or other materials used to remove the SO₂ from waste gases are alkaline. A typical reaction taking place in wet scrubbing using a CaCO₃ (limestone) slurry produces CaSO₃ (calcium sulfite) and can be expressed as: CaCO₃(solid)+SO₂(gas)→CaSO₃(solid)+CO₂(gas).

When wet scrubbing with a Ca(OH)₂ (lime) slurry, the reaction also produces CaSO₃ (calcium sulfite) and can be expressed as: Ca(OH)₂(solid)+SO₂(gas)→CaSO₃(solid)+H₂O(liquid).

When wet scrubbing with a Mg(OH)₂ (magnesium hydroxide) slurry, the reaction produces MgSO₃ (magnesium sulfite) and can be expressed as: Mg(OH)₂(solid)+SO₂(gas)→MgSO₃(solid)+H₂O(liquid).

The CaSO₃ (calcium sulfite) can be further oxidized to produce marketable CaSO₄. 2H₂O (gypsum). This technique is also known as forced oxidation: CaSO₃(solid)+H₂O(liquid)+½O₂(gas)→CaSO₄(solid)+H₂O.

A common wet limestone FGD system removes sulfur dioxide from the flue gas by passing the flue gas through a chamber that exposes the flue gas to a spray slurry of finely ground limestone. The slurry absorbs the SO₂ from the flue gas by reaction of the calcium in the limestone with SO₂ to form a mixture of calcium sulfite and calcium sulfate. The slurry is pumped through banks of countercurrent spray nozzles, creating fine droplets with uniform contact with the flue gas.

After absorbing the sulfur dioxide from the flue gas, the slurry collects in the bottom of the absorber in a reaction tank. It is aerated via forced oxidation to oxidize bisulfite ion to sulfate. This can avoid formation of calcium sulfite hemihydrates in favor of producing calcium sulfate dihydrate, or gypsum, which precipitates. Oxidized slurry is then recirculated to the spray headers. A portion of the slurry is withdrawn to remove the precipitated gypsum. Typically, the slurry is dewatered in a two-stage process involving a hydroclone and vacuum filter system to produce a gypsum cake for disposal or sale. Water removed from the slurry is returned to the process. A portion of this water is removed from the system as wastewater to limit the accumulation of chloride salts and other undesirable constituents introduced with the coal.

The solid fines are separated from the coarse gypsum solids using a hydroclone. The hydroclone overflow stream is further treated prior to wastewater discharge and return of a portion of the liquid to the slurry reaction tank.

Other scrubbing reagents can be used in the scrubbing solution. As explained above, alkaline sorbents are used for scrubbing flue gases to remove SO₂. Other examples of alkaline sorbents include sodium hydroxide (also known as caustic soda). Caustic soda has the advantage that it forms a solution rather than a slurry. It produces a “spent caustic” solution of sodium sulfite/bisulfite (depending on the pH), or sodium sulfate that must be disposed of. It is possible to scrub sulfur dioxide by using a cold solution of sodium sulfite, this forms a sodium hydrogen sulfite solution. By heating this solution it is possible to reverse the reaction to form sulfur dioxide and the sodium sulfite solution. Since the sodium sulfite solution is not consumed, it is called a regenerative treatment. The application of this reaction is also known as the Wellman-Lord process.

The further treated gas stream 136 is emitted, via gas discharge or stack 140, into the environment.

The Iodine-Containing Additive for Mercury Control

To control mercury emissions in the mercury-containing gas stream 108, an iodine-containing additive 144, typically in the form of iodide salts or as iodine or iodate (IO₃ ⁻), can be employed. The iodine in the additive 144 can be in the form of a solid, liquid, vapor, or a combination thereof. It can be in the form of an elemental halogen (e.g., iodine (I₂)), a halide (e.g., binary halides, oxo halides, hydroxo halides, and other complex halides), an inter-halogen cation or anion, a haloacid (e.g., iodic acid and periodic acid), a halosalt (e.g., a periodate), a homoatomic polyanion, and mixtures thereof. In one formulation, the iodine in the additive 144 is composed primarily of an alkali or alkaline earth metal iodide. In one formulation, the iodine-containing additive 144 is substantially free of other halogens and even more typically contains no more than about 25%, even more typically no more than about 10%, and even more typically no more than about 5% of the halogens as halogen(s) other than iodine. In one formulation, the iodine-containing additive 144 contains at least about 100 ppmw, more commonly at least about 1,000 ppmw, and even more commonly at least about 1 wt. % iodine. In one formulation, the iodine-containing additive contains no more than about 40 wt. % fixed or total carbon, more commonly no more than about 25 wt. % fixed or total carbon, even more commonly no more than about 15 wt. % fixed or total carbon, and even more commonly no more than about 5 wt. % fixed or total carbon. In one formulation, the iodine-containing additive 144 is a high (native) iodine coal. In one formulation, the iodine-containing additive 144 is an iodine-containing waste or byproduct material, such as a medical waste. In one formulation, the iodine-containing additive 144 comprises iodine attached to a solid support, such as by absorption, adsorption, ion exchange, formation of a chemical composition, precipitation, physical entrapment, or other attachment mechanism. The solid support can be inorganic or organic. Examples include ion exchange resins (functionalized porous or gel polymers), soil humus, a porous carbonaceous material, metal oxides (e.g., alumina, silica, silica-alumina, gamma-alumina, activated alumina, acidified alumina, and titania), metal oxides containing labile metal anions (such as aluminum oxychloride), non-oxide refractories (e.g., titanium nitride, silicon nitride, and silicon carbide), diatomaceous earth, mullite, porous polymeric materials, crystalline aluminosilicates such as zeolites (synthetic or naturally occurring), amorphous silica-alumina, minerals and clays (e.g., bentonite, smectite, kaolin, dolomite, montmorillinite, and their derivatives), porous ceramics metal silicate materials and minerals (e.g., one of the phosphate and oxide classes), ferric salts, and fibrous materials (including synthetic (for example, without limitation, polyolefins, polyesters, polyamides, polyacrylates, and combinations thereof) and natural (such as, without limitation, plant-based fibers, animal-based fibers, inorganic-based fibers, cellulosic, cotton, paper, glass and combinations thereof). Commonly, the iodine-containing additive 144 contains no more than about 10 wt. % iodine, more commonly no more than about 5 wt. % iodine, even more commonly no more than about 1 wt. % iodine, even more commonly no more than about 0.5 wt. % iodine, and even more commonly no more than about 0.1 wt. % iodine.

The iodine-containing additive 144 can be contacted with the mercury-containing gas stream 108 at any of a variety of locations, including upstream of the economizer 112, in the economizer 112, at the inlet of the SCR 116, in the SCR 116, at the inlet of the air preheater 120, in the air preheater 120, at the inlet of the particulate removal device 124, and in the particulate removal device 124. It can also be added to the feed material 100 prior to and/or during combustion. For example, the iodine-containing additive 144 can be added directly to the feed material 100 upstream of the thermal unit 104. The coal feed can be treated with a potassium iodide solution or a carbonaceous sorbent enriched in iodine can be added to the coal. Alternatively, the iodine may be injected into the boiler combustion zone or the process off-gas stream. The range of addition is typically about 1 to 30 ppm_(w) of the coal feed. When introduced into the mercury-containing gas stream 108, the iodine-containing additive 144 is introduced into the gas stream 108, such as by injection as a liquid, vapor, or solid powder. The iodine-containing additive can be dissolved in a liquid, commonly aqueous, in the form of a vapor, in the form of an aerosol, or in the form of a solid or supported on a solid. In one formulation, the iodine-containing additive 144 is introduced as a liquid droplet or aerosol downstream of the thermal unit 104. In this formulation, the iodine is dissolved in a solvent that evaporates, leaving behind solid or liquid particles of the iodine-containing additive 144.

In one plant configuration, sufficient iodine-containing additive 144 is added to produce a gas-phase iodine concentration commonly of about 8 ppm_(w) basis of the flue gas or less, even more commonly of about 5 ppm_(w) basis or less, even more commonly of about 3.5 ppm_(w), basis or less, even more commonly of about 1.5 ppm_(w) or less, and even more commonly of about 0.4 ppm_(w) or less of the mercury-containing gas stream. Stated another way, the iodine concentration relative to the weight of mercury-containing, combustible (e.g., coal) feed (as fed) (whether by direct application to the combustible feed and/or injection into the mercury-containing (e.g., flue) gas) commonly is about 40 ppmw or less, more commonly about 35 ppmw or less, even more commonly about 30 ppmw or less, even more commonly is about 15 ppmw or less, even more commonly no more than about 10 ppmw, even more commonly no more than about 6 ppmw, even more commonly about 4 ppmw or less, and even more commonly no more than about 3 ppmw. Stated another way, the molar ratio, in the mercury-containing (e.g., flue) gas, of gas-phase diatomic iodine to total gas-phase mercury (both speciated and elemental) is commonly no more than about 1,200, and even more commonly no more than about 600, even more commonly no more than about 250, even more commonly no more than about 150, and even more commonly no more than about 80. By way of illustration, an effective concentration of gas-phase iodine at the air preheater outlet or particulate removal device inlet ranges from about 0.1 to about 10 ppmw, even more commonly from about 0.15 to about 5 ppmw, even more commonly from about 0.20 to about 2 ppmw, and even more commonly from about 0.25 to about 1.50 ppmw of the mercury-containing gas stream.

Commonly, the mercury-containing gas stream includes no more than about 1.0, even more commonly no more than about 0.5 and even more commonly no more than about 0.1 ppmw total bromine. The feed material generally includes no more than about 10 ppmw and even more commonly no more than about 5 ppmw natively occurring bromine.

The mercury-containing (e.g., flue) gas temperature for elemental mercury capture promoted by iodine commonly ranges from about 150 to about 600° C. and even more commonly from about 180 to about 450° C. The residence time upstream of particulate (e.g., fly ash) removal device 120 is commonly about 8 seconds, and even more commonly at least about 4 seconds, and even more commonly at least about 2 seconds.

Generally, sufficient iodine-containing additive 144 is added to produce a gas-phase iodine concentration commonly of about 3.5 ppmw or less, even more commonly of about 2 ppmw or less, even more commonly of about 1.5 ppmw or less, and even more commonly of about 0.4 ppmw or less. Stated another way, the molar ratio, in the mercury-containing (e.g., flue) gas, of gas-phase iodine to total gas-phase mercury (both speciated and elemental) is commonly no more than about 1,000, even more commonly no more than about 600, even more commonly no more than about 500, even more commonly no more than about 250, even more commonly no more than about 150, and even more commonly no more than about 80.

The Carbonaceous Material

The carbonaceous material 150 can be any porous (preferably microporous) carbonaceous iodine sorbent, including (powdered, granular, extruded, bead, impregnated, or polymer coated) activated carbon, carbon black, char, charcoal, and pet coke. The carbonaceous material 150 commonly has an iodine number ranging from about 250 to about 1,500 mg/g, more commonly from about 300 to about 1,450 mg/g, more commonly from about 350 to about 1,400 mg/g, more commonly from about 350 to about 1,350 mg/g, and even more commonly from about 500 to about 1,300 mg/g.

While the carbonaceous material 150 can (as shown by the arrows) be added at any location or multiple locations in or upstream of the particulate removal device 124 to remove mercury from the waste gas (the iodine-containing additive 144 can promote both mercury oxidation and chemisorption on the carbonaceous material 150), it is contacted with the contaminated scrubbing solution 160 in the wet scrubber 132 to control iodine levels, which can impair scrubber performance. Stated another way, the carbonaceous material can be injected in multiple locations simultaneously, for example, into the fabric filter inlet to boost oxidation and into the wastewater treatment to recover iodine.

By way of illustration, uncontrolled iodide buildup in solution can occur in a recirculating wet FGD scrubber, thereby interfering with SO₂ absorption. When SO₂ is absorbed in water the chemical reaction forms bisulfite, which is a well-known reducing agent: SO₂+H₂O→H⁺+HSO₃ ⁻ SO₂-absorption

In typical FGD scrubbers designed for gypsum production, this bisulfite ion is oxidized to sulfate by molecular oxygen. HSO₃ ⁻+½O₂→H⁺+SO₄ ⁻² Sulfite oxidation

Sulfite concentration in the scrubbing (slurry) solution controls the rate of SO₂ absorption. Low sulfite concentration in the scrubbing solution is one measure of high oxidation. If the oxidation rate is too low, the SO₂ removal efficiency can be decreased.

Iodine in the scrubbing solution is an oxidation inhibitor. High concentrations of iodide/iodine in the recirculated scrubbing solution moderate the sulfite oxidation rate and suppress the oxidation-reduction potential (ORP). Iodide is able to reduce either the sulfite or the peroxomonosulfate radicals or the catalytically active transition-metal ions and is thereby oxidized to iodine. It can subsequently be reduced to iodide again by excess sulfite. Therefore, iodine is able to inhibit the overall SO₂ to sulfating reaction(s) and is not consumed in the process.

Buildup of iodine and iodide in the recirculating scrubbing solution can have other effects.

Lower ORP as a result of iodide can beneficially cause speciation of selenium in scrubber solution from selenate (SeO₄ ²⁻) to selenite (SeO₃ ²⁻). Selenate is extremely difficult to remove prior to discharge and therefore, for some scrubbers that are limited by wastewater discharge limits for selenium, ORP reduction in a controlled manner via control of iodide concentration could be a desirable outcome.

Gas-phase mercury in coal combustion flue gases is present as either elemental)(Hg⁰), or oxidized (ionic) mercury (Hg⁺⁺). Elemental mercury is relatively insoluble in aqueous solutions, and is not removed at significant percentages in wet FGD absorbers. Oxidized mercury as mercuric chloride or mercuric bromide is very soluble and is absorbed into the FGD scrubbing slurry at high efficiency. Mercuric iodide, however is not as soluble. Oxidized mercury iodide can be lost from FGD slurry solution when utilizing iodine as a coal additive for mercury control.

The net removal of oxidized mercury across the FGD system is often limited to lower percentages by mercury “re-emission”. While not wishing to be bound by any theory, a portion of the absorbed oxidized mercury can undergo chemical reduction reactions while dissolved in the aqueous phase and be converted to the relatively insoluble elemental form. Once converted, it is released from the aqueous phase back into the further treated waste gas 136.

An increase in vapor-phase iodine in the waste gas 108 upstream of a wet FGD scrubber can also lead to undesirable mercury re-emission from the wet scrubber. As noted, iodine suppresses ORP in slurry solution. Scrubber sulfite in turn is increased to compensate and Hg⁺⁺ species dissolved in solution may then be reduced to Hg⁰ and re-emitted. Iodine is also reduced to iodide by sulfite and therefore the iodide concentration in solution will be increased.

Commonly, the carbonaceous material 150 contacted with the scrubbing solution maintains a concentration of iodide and iodine in the scrubbing solution of no more than about 1,000 mg/L, more commonly of no more than about 500 mg/L, and even more commonly of no more than about 50 mg/L. A typical concentration range of iodine and iodide in the scrubbing solution is from about 0.01 to about 50 mg/L, more typically from about 0.05 to about 25 mg/L, and even more typically from about 0.10 to about 5 mg/L. In some applications, a flow of the scrubbing solution through a fluidized or fixed packed bed of the carbonaceous material is controlled at a rate to maintain overall iodine concentration in the scrubber solution commonly at less than about 100 ppm and more commonly at less than 50 ppm.

While the iodide concentration in the recirculating scrubbing solution can be indirectly controlled through periodic scrubber blow down and waste water discharge, this is typically not effective in the absence of contacting the recirculating scrubbing solution with a carbonaceous material. As will be appreciated, “blow down” is a method of solids control. In scrubbers, solids accumulation (both dissolved and suspended solids) can interfere with the operation of the unit. Solids accumulate because of evaporative processes, and, to control the solids buildup. The solids buildup tend to be particulate when the scrubber is operating on the waste gas, and salts when the scrubber is operating on a chemical stream off gas. The salt buildup beyond a certain point can reduce the efficiency of the scrubber by approaching the solubility point of the scrubbed material in the scrubber solution. When the solubility limit is reached, the absorption stops and the scrubber efficiency can drop to almost zero. Alternatively, the scrubber may, when the solubility limit is reached, begin to deposit solids on the walls of the pipe and the like. To reduce the solids accumulation, the scrubber water rich in solids or salts is wasted or blown down and replaced with low solids water.

Blow down frequency is often adjusted to control total dissolved solids (TDS) and/or chlorides but also can incidentally reduce iodide concentration. Regular blow down schedule may be insufficient when high levels of inlet flue gas iodine are maintained over a long period of time.

The carbonaceous material 150 can remove commonly at least most, more commonly at least about 65%, more commonly at least about 75%, and even more commonly at least about 85% of the iodine and iodine-containing compounds in the scrubber solution from the treated waste stream 128. As a result of iodine removal, the further treated waste gas 136 has a low concentration of the iodine and/or iodine-containing compounds. Preferably, the concentration of the iodine and/or iodine-containing compounds in the further treated waste gas 136 is no more than about 25 ppm, more preferably no more than about 20 ppm, more preferably no more than about 15 ppm, more preferably no more than about 10 ppm, and even more preferably no more than about 5 ppm.

Total carbonaceous sorbent required for iodine removal can be relatively minimal. Powdered activated carbon was laboratory tested for iodine uptake from brine solution. In these tests, the iodine source was potassium iodide that was oxidized in solution with dilute hydrogen peroxide. Uptake of up to 100% as iodine by weight of the carbon was measured. The activated carbon tested was a microporous high capacity with an iodine number of >800 mg/g. In practice, it is expected that working capacity of about 0.5 lb/lb of carbonaceous sorbent could be achieved. As an example, for a 500 MW power plant with iodine coal additive for mercury control, about 3 to 15 lbs/hr of a highly microporous activated carbon would be sufficient to recover all of the coal additive. The annual recoverable product would be about 3 to 12 tons/year, assuming a 50% recovery.

Iodine Recovery, Regeneration, and Recycle

Returning to FIGS. 1 and 6, an optional regeneration facility 170 can recover iodine-impregnated carbonaceous material 150 from the scrubbing solution and recirculate the recovered iodine-impregnated carbonaceous material 150 as the iodine-containing additive 144. While not wishing to be bound by any theory, it is believed that mercury and other volatile metals that were present in the scrubbing solution are retained on the carbonaceous material at very low concentration compared to the mercury content of the input coal. Therefore, the carbonaceous material with sorbed iodine can be charged back to the coal belt and combusted without concern of overall mercury buildup in the combustion process. In this way, a portion of the iodine required as coal additive can be recovered directly from the process without the need for purchase of expensive and overly purified primary iodine supply. Removal of iodide from the waste water recycle, combined with the normal blow down process, will commonly keep iodide concentration in the scrubbing solution at low concentration. This in turn will increase retention of the inlet iodine (and better overall recovery of iodine) in the liquid phase by maintaining the concentration farther below a solubility limit.

The operation of the facility 170 will be discussed with reference to FIG. 6.

In step 600, the waste gas 108 is contacted with the scrubbing solution in the wet scrubber 132 to form a contaminated scrubbing solution. While the contents can vary by application and scrubbing reagent employed, the contaminated scrubbing solution will typically include, in addition to bisulfite ion, sulfite ion, sulfate ion, selenite, selenate, iodine, iodide, sulfuric acid, carbonate, and gypsum.

In step 604, the contaminated scrubbing solution is contacted with the carbonaceous material 150. Contacting may be effected by passing the contaminated scrubbing solution through a packed or fluidized bed, forming a slurry containing the carbonaceous material 150 (such as by introducing the carbonaceous material into the waste gas scrubber inlet and/or directly into the scrubbing solution), as a separate treatment step of the fine solids in the scrubber wastewater treatment stream, and as a polishing step to recover iodine from recycle water or wastewater discharge.

In optional steps 608 and 612, the contaminated scrubbing solution is contacted with an oxidizing agent, such as a hypochlorite, and/or pH-adjusted by contact with a pH adjustor to convert at least most and commonly substantially all iodide to iodine and oxidize mercury sorbed onto the carbonaceous material to a soluble form of mercury, such as Hg⁺⁺ species dissolved in solution. Iodine is sorbed more readily by the carbonaceous agent than iodide, and mercury oxidation can remove at least most and commonly substantially all of the sorbed mercury from the surface of the carbonaceous material. Commonly, the ORP of the contaminated scrubbing solution for a limestone forced oxidation scrubber employing recirculation is maintained in the range of from about 200 to about 800 mV (as measured typically platinum/gold ORP electrode) and more commonly from about 300 to about 700 mV (as measured by typically platinum/gold ORP electrode), and the pH is maintained in the range of from about pH 5 to about pH 6 and more commonly from about pH 5 to about pH 5.5.

The dissolved mercury can be precipitated by a suitable precipitant from the contaminated scrubbing solution. Suitable precipitants include sulfides, particularly organosulfides, or an amalgamating agent.

In optional step 616, the iodine-loaded carbonaceous material is recycled to the thermal unit 104 and/or introduced into the waste gas 108 to enhance elemental and speciated mercury removal.

Referring to FIG. 2, an exemplary iodine recovery, regeneration, and recycle plant configuration is depicted. In the configuration, contaminated scrubbing solution 200 is removed from the wet scrubber 132, passed through a fixed or fluidized bed of carbonaceous material 150 and the treated scrubbing solution 204 returned to the scrubber 132.

FIG. 5 depicts a regeneration facility 170 that may be employed in the plant configuration of FIG. 2.

The facility 500 includes an input conduit 504 to carry contaminated scrubbing solution, an input sensor 508 (such as a spectrophotometer) to sense the concentration of iodine and/or iodide in the contaminated scrubbing solution, an input manifold 512 and plural controllable input valves 516 a, b, . . . to direct the contaminated scrubbing solution to an appropriate bed of carbonaceous material in a selected vessel 520 a, b, . . . , plural controllable output valves 524 a, b, . . . to prevent backflow of treated contaminated scrubbing solution to other non-selected beds, an output manifold 528, an output conduit 532 to carry the treated contaminated scrubbing solution back to the scrubber, and an output sensor 536 (such as a spectrophotometer) to sense the concentration of iodine and/or iodide in the treated contaminated scrubbing solution.

The facility further includes a controller 540 in communication, via control lines 544, 548, 552, and 556, with the input and output sensors 508 and 536 and input and output valves 516 a, b, . . . and 524 a, b, . . . . The controller 540 comprises a computer readable medium (not shown) and microcontroller (not shown) to sense an input iodine and/or iodide concentration in the contaminated scrubbing solution and output iodine and/or iodide concentration in the treated contaminated scrubbing solution, compare the input and output concentrations, when the output concentration and/or the difference between the input and output concentrations is at least a specified threshold, determine that the currently selected bed of carbonaceous material requires regeneration or replacement, and in response, close the input and output valves to the vessel containing the bed, and open the input and output valves to the vessel containing a next selected bed.

The carbonaceous material from the spent bed can be regenerated and/or recycled as the iodine-containing additive 144. Regeneration could include steps 608 and 612 to remove sorbed mercury followed by a suitable process to desorb iodine and iodide from the carbonaceous material. For example, iodine is soluble in, and desorbed from the carbonaceous material in, a number of organic solvents. In another example, the carbonaceous material can be acidified with a mineral acid to a pH in the range of about pH 1 to about pH 4 to dissolve the iodine in solution. In another example, the carbonaceous material can be reduced by a reducing agent to iodide, which has less affinity for the carbonaceous material. Under proper conditions of pH, the iodide will desorb into solution. Another technique is by elution from the carbonaceous material using aqueous sodium hydroxide. In another technique, the iodine is eluted by contacting the carbonaceous material with aqueous sodium chloride (NaCl) and bleach (NaOCl) under slightly acidic pH. Once dissolved, the iodine can be isolated by known techniques, such as precipitation, an ion exchange resin, and the like.

FIG. 3 depicts a scrubbing facility that may be employed in the plant configuration of FIG. 1. The wet scrubber 132 includes a further treated waste gas 136 outlet 304, a scrubber housing 308, a scrubbing solution 300 inlet 312 (which is typically a series of spray nozzles to provide an atomized spray of scrubbing solution 300), and a scrubbing solution 300 outlet 316, and treated waste gas 128 inlet 320. The scrubber can include one or more scrubbing vanes (not shown) to induce turbulent gas flow within the scrubber housing 308. The contaminated scrubbing solution 328 settles in a lower portion 324 of the scrubber 132 for removal by the scrubbing slurry outlet 316. An oxidizing agent 332 can be introduced to perform forced oxidation as discussed above.

The contaminated scrubbing solution 328 is removed from the scrubber 132 and sent to a dewatering plant 336. A typical dewatering plant includes one or more of hydrocyclones, filters, settling tanks, and the like, to dewater the solid byproduct in the contaminated scrubbing solution 328. The solid byproduct can, for instance, be a carbonate, hydroxide, sulfate (e.g., gypsum), sulfite, fly ash, and other recovered waste gas constituents.

The dewatering plant 332 produces contaminated water 340 and a solid byproduct 344.

The contaminated water 340 is contacted with the carbonaceous material 150, such as by passing the water 340 through a fixed or fluidized bed of carbonaceous material 150. When the carbonaceous material 150 is loaded with iodine and/or iodide, it can be provided to the regeneration facility 170 to produce clean carbonaceous material 348 substantially free of elemental and speciated mercury and iodine, a mercury byproduct 352, and an iodine-containing product 356. Alternatively, after mercury removal the carbonaceous material 150 can be used as an iodine-containing additive 144.

The treated water 360 is subjected to oxidation and/or pH adjustment (box 364) to precipitate waste gas contaminants (which are then removed by a liquid/solid separation process (not shown), and the purified water 368 combined, in a mixing vessel 372, with fresh water and additional scrubbing reagent, to provide recirculated scrubbing solution 300.

FIG. 4 depicts a process for recovering carbonaceous material introduced upstream of or into the treated gas inlet of the scrubber. In that event, the carbonaceous material and scrubbing solution form a slurry or the carbonaceous material becomes part of a slurried scrubbing solution, particularly an FGD scrubber producing a gypsum byproduct. Following oxidation (step 608) and pH adjustment (step 612), the resulting contaminated scrubber slurry 400 is subjected to separation of the carbonaceous material from the other particulates (such as gypsum and/or flyash) to form a recovered carbonaceous material 150 and other particulates 408. The separation may be based on the differing properties of the carbonaceous material 150 and other particulates 408. The properties include particle size, density, weight, specific gravity, and the like. For example, filtration may be used to effect the separation where the carbonaceous material 150 and other particulates 408 have different particles sizes and size fractions. Hydrocyclones may be used to effect separation where the carbonaceous material 150 and other particulates 408 have different weights, specific gravities, and/or densities. Gravity settling and/or flotation may be used where the carbonaceous material 150 and other particulates 408 have different hydrophilicities and/or hydrophobicities.

In one configuration, the solid fines containing suspended carbonaceous material are separated from the coarse gypsum solids using a hydroclone. The hydroclone overflow stream with the fines in liquid suspension is next treated by contact with an oxidizing agent, such as sodium hypochlorite, to remove collected mercury from the activated carbon. At this step, the mercury is redissolved into solution and the activated carbon is substantially mercury-free. The mercury is precipitated from solution by contact with organosulfides.

The hydroclone overflow stream can be treated with oxidizer and pH adjusted, as necessary, to oxidize iodide to iodine. Iodine is then efficiently adsorbed on the suspended carbonaceous material, leaving a very low concentration in solution. The carbonaceous material is then separated from solution by filtration and dewatered.

Another process configuration is introduction of carbonaceous sorbent expressly for iodine recovery into the hydroclone overflow stream in combination with an oxidizer or oxidizing agent.

Other iodide/iodine recovery methods are possible. In one configuration, the recycle wastewater and/or discharge wastewater are passed through a final polishing treatment wherein an oxidizer such as hydrogen peroxide, potassium permanganate, chlorine or sodium hypochlorite is added and the treated stream is then passed through a packed granular activated carbon bed. This step would follow any treatment for removal of metals such as mercury or selenium. The packed bed is then changed when fully loaded, the carbon can be dried and then recharged to the coal belt. Alternatively, iodine could be recovered from a carbonaceous sorbent or granular bed by further treatment steps to a purified commercial product if there was sufficient demand and it was cost effective to do so.

Another method that can be used for direct iodide removal from aqueous solution is the use of ion (anion) exchange, typically with proprietary organic ion exchange resins. Anion exchange would typically be utilized after separation of coarse solids from the slurry treatment loop. The iodine and/or iodine-containing compounds removed by ion exchange can be stripped from the ion exchange medium by a stripping solution.

Dissolved iodine and/or iodine-containing compounds can be removed from the scrubbing solution by other techniques, including solvent extraction, adsorption, absorption, precipitation, membrane filtration, and the like. In one configuration, the dissolved iodine and/or iodine-containing compounds are removed in an organic solvent, such as a hydrocarbon solvent. Removal is effected based on the relative solubilities of the iodine and/or iodine-containing compounds in two different immiscible liquids, namely the aqueous phase of the scrubbing solution and an organic solvent. In other words, the iodine and/or iodine-containing compounds are extracted from one liquid phase into another liquid phase. Extraction can be done without chemical change, by a solvation mechanism, by an ion exchange mechanism, by ion pair extraction, or by aqueous two-phase extraction.

Other dissolved impurities, such as sulfur oxides, nitrogen oxides, and the like, can be removed before recycle. Selective removal of the various species can be, for example, by membrane separation, precipitation, adsorption, and/or absorption.

Upstream Oxidation of Mercury Species

Gaseous oxidized mercury compounds are much more soluble in water than elemental mercury. Mercury entering a wet FGD scrubber should ideally be fully or substantially fully oxidized to maximize capture of mercury in the scrubber. Halogen additives increase mercury oxidation and also retention on fly ash and Loss on Ignition (“LOI”) carbon particulate surfaces upstream of the scrubber. With reference to FIGS. 1 and 2, if a fabric filter baghouse is installed, as the particulate removal device 124, wet upstream of the scrubber 132, mercury oxidation can be increased during flue gas passage through the fabric filter and ash cake. Carbonaceous sorbents injected into and/or upstream of the baghouse can further increase mercury oxidation. Typically, the total mercury is removed to a high percentage across the fabric filter, and the vapor mercury exits the baghouse as Hg⁺⁺.

High sulfur dioxide and SO₃ or other acid flue gas species such as NO₂ inhibit mercury capture on fly ash or carbonaceous sorbents in the baghouse, but do not inhibit mercury oxidation. An increase in mercury oxidation is still beneficial for downstream mercury capture in the scrubber. Mercury oxidation across a fabric filter can be enhanced by selection of bag fabric, temperature and ash cake management, sorbent injection, oxidation catalyst or halogen coal additive. This can allow the treatment rate of initial halogen coal additive to be reduced or even eliminated when the final mercury control device is a wet scrubber.

EXPERIMENTAL

The following examples are provided to illustrate certain aspects, embodiments, and configurations of the disclosure and are not to be construed as limitations on the disclosure, as set forth in the appended claims. All parts and percentages are by weight unless otherwise specified.

Short-term tests of liquid iodide salt solution added to the coal for mercury reduction have been conducted on a full-scale boiler firing a PRB/bituminous blend coal and equipped with a LSFO scrubber. An increase in mercury re-emission was evidenced when the coal additive was applied. This test confirms the short-term impact of iodide increase in the FGD. Over time, it is expected that this could be prevented by extraction of iodide from slurry solution.

Powdered activated carbon has been tested in the laboratory for iodine uptake from brine solution. In these tests, the iodine source was potassium iodide that was oxidized by dilute hydrogen peroxide. Uptake of up to 100% as iodine by weight of the carbon was measured.

Scrubber ORP was monitored over a period of a month at a Midwestern power plant firing a PRB coal and employing a limestone forced oxidation scrubber for sulfur control. Iodine was added onto the coal at low ppm level as a mercury control agent over the entire monitored period. Iodine in solution (as iodide) increased from 0 to about 46 ppm over about 24 days. Iodine in the slurry solids, measured after separation from the slurry liquid, increased from about 20 ppm to 645 ppm after 15 days. This indicates that the majority of iodine is partitioning to the solids as insoluble iodine. However, no attempt was made during this test to separate iodine from the scrubber liquor. The use of carbon sorbents or other separation technologies, as described herein, could have allowed the iodine as I2 to separate onto the carbon sorbent preferentially to the gypsum solids. Separation would have reduced the total iodine concentration in solution over time, since iodide species are both oxidized in the scrubber absorber as well as reduced back to iodide.

A number of variations and modifications of the disclosure can be used. It would be possible to provide for some features of the disclosure without providing others.

For example in one alternative embodiment, the present disclosure is not limited to a recirculating scrubber. It may be applied to other types of non-recirculating scrubbers.

In another alternative embodiment, the present disclosure is employed with any of a number of types of wet scrubbers. To promote maximum gas-liquid surface area and residence time, a number of wet scrubber designs have been used, including spray towers, venturis, plate towers, and mobile packed beds. The configuration of the tower may be vertical or horizontal, and flue gas can flow cocurrently, countercurrently, or crosscurrently with respect to the liquid. A venturi scrubber is a converging/diverging section of duct. The converging section accelerates the gas stream to high velocity. When the liquid stream is injected at the throat, which is the point of maximum velocity, the turbulence caused by the high gas velocity atomizes the liquid into small droplets, which creates the surface area necessary for mass transfer to take place. The higher the pressure drop in the venturi, the smaller the droplets and the higher the surface area. A packed scrubber includes a tower with packing material inside. This packing material can be in the shape of saddles, rings, or some highly specialized shapes designed to maximize contact area between the dirty gas and liquid. A spray tower includes a tower with spray nozzles, which generate the droplets for surface contact. Spray towers are typically used when circulating a slurry (see below). The high speed of a venturi would cause erosion problems, while a packed tower would plug up if it tried to circulate a slurry.

In yet other embodiments, the sequence of the process steps in any of FIGS. 4 and 6 can be reversed and/or the steps can be combined, depending on the application.

In yet other embodiments, mercury is oxidized by introducing, on the feed material and/or in the waste gas, a halogen and/or halogen-containing material in lieu of or addition to iodine and/or iodine-containing material. The halogen and/or halogen-containing material can be, for example, bromine, chlorine, bromide, chloride, and other bromine- and/or chlorine-containing compounds or compositions.

The present disclosure, in various aspects, embodiments, and configurations, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various aspects, embodiments, configurations, subcombinations, and subsets thereof. Those of skill in the art will understand how to make and use the various aspects, aspects, embodiments, and configurations, after understanding the present disclosure. The present disclosure, in various aspects, embodiments, and configurations, includes providing devices and processes in the absence of items not depicted and/or described herein or in various aspects, embodiments, and configurations hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving ease and\or reducing cost of implementation.

The foregoing discussion of the disclosure has been presented for purposes of illustration and description. The foregoing is not intended to limit the disclosure to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the disclosure are grouped together in one or more, aspects, embodiments, and configurations for the purpose of streamlining the disclosure. The features of the aspects, embodiments, and configurations of the disclosure may be combined in alternate aspects, embodiments, and configurations other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the claimed disclosure requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed aspects, embodiments, and configurations. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the disclosure.

Moreover, though the description of the disclosure has included description of one or more aspects, embodiments, or configurations and certain variations and modifications, other variations, combinations, and modifications are within the scope of the disclosure, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative aspects, embodiments, and configurations to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter. 

The invention claimed is:
 1. A method, comprising: contacting, upstream of a wet scrubber, a halogen-containing additive with a waste gas comprising an acid gas and elemental mercury to oxidize the elemental mercury in the waste gas and form a treated gas stream comprising a non-elemental mercury and the halogen-containing additive; passing the treated gas stream through the wet scrubber to remove the halogen-containing additive from the waste gas and form a scrubbing solution comprising the halogen-containing additive; and sorbing at least a portion of the halogen-containing additive on a carbonaceous material.
 2. The method of claim 1, further comprising: contacting the halogen-containing additive with a mercury-containing feed material, wherein the waste gas is derived from the mercury-containing feed material.
 3. The method of claim 1, further comprising: recovering the carbonaceous material comprising the sorbed at least a portion of the halogen-containing additive from the scrubbing solution; and introducing the recovered the carbonaceous material comprising the sorbed at least a portion of the halogen-containing additive to the waste gas as the halogen-containing additive.
 4. The method of claim 1, further comprising: separating the sorbed at least a portion of the halogen-containing additive from the carbonaceous material; and introducing the separated at least a portion of the halogen-containing additive to the waste gas as the halogen-containing additive.
 5. The method of claim 1, recovering the carbonaceous material comprising the sorbed at least a portion of the halogen-containing additive from the scrubbing solution; and returning the scrubbing solution to the wet scrubber and wherein the wet scrubber is a flue-gas desulfurization scrubber.
 6. The method of claim 1, wherein a flow of an input scrubbing solution through a first bed of carbonaceous material is stopped when a concentration of the halogen-containing additive in an output scrubbing solution is at least a predetermined threshold and/or a difference between concentrations of the halogen-containing additive in the input and output scrubbing solutions is at least a predetermined threshold.
 7. The method of claim 6, wherein the flow through the input scrubbing solution through the first bed of carbonaceous material is stopped automatically by a microprocessor and a flow of the output scrubbing solution through a second bed of carbonaceous material is initiated automatically by the microprocessor in response to an absolute value of the concentration of the halogen-containing additive in the output scrubbing solution reaching the at least a predetermined threshold.
 8. The method of claim 1, wherein the carbonaceous material is activated carbon and wherein the carbonaceous material is introduced into a reservoir of the wet scrubber.
 9. The method of claim 1, wherein the carbonaceous material is activated carbon and wherein the carbonaceous material is introduced upstream of the wet scrubber.
 10. The method of claim 1, further comprising contacting the scrubbing solution with one or more of an oxidizing agent and a pH-adjustor.
 11. A method, comprising: contacting, upstream of a wet scrubber, a halogen-containing additive with a waste gas comprising an acid gas and elemental mercury to form a treated gas stream comprising the acid gas, a non-elemental mercury oxidized from the elemental mercury, and at least one of an elemental halogen and a halogen-containing composition; passing the treated gas stream through the wet scrubber to remove the at least one of the elemental halogen and the halogen-containing composition from the waste gas and form a scrubbing solution comprising the at least one of the elemental halogen and the halogen-containing composition; sorbing at least a portion of the at least one of the elemental halogen and the halogen-containing composition on a carbonaceous material; and recovering the carbonaceous material comprising the sorbed at least a portion of the at least one of the elemental halogen and the halogen-containing composition from the scrubbing solution wherein the recovered carbonaceous material is one or more of: (i) introduced to the waste gas as the halogen-containing additive; and (ii) charged to coal and, thereafter, combusted with the coal to form the waste gas.
 12. The method of claim 11, further comprising: contacting the halogen-containing additive with a mercury-containing feed material, wherein the waste gas is derived from the mercury-containing feed material.
 13. The method of claim 11, wherein after the recovering, the scrubbing solution is returned to the wet scrubber and wherein the wet scrubber is a flue-gas desulfurization scrubber.
 14. The method of claim 11, wherein a flow of an input scrubbing solution through a first bed of carbonaceous material is stopped when a concentration of the at least one of the elemental halogen and the halogen-containing composition in an output scrubbing solution is at least a predetermined threshold and/or a difference between concentrations of the at least one of the elemental halogen and the halogen-containing composition in the input and output scrubbing solutions is at least a predetermined threshold.
 15. The method of claim 14, wherein the flow through the input scrubbing solution through the first bed of carbonaceous material is stopped automatically by a microprocessor and a flow of the output scrubbing solution through a second bed of carbonaceous material is initiated automatically by the microprocessor in response to an absolute value of the concentration of the at least one of the elemental halogen and the halogen-containing composition in the output scrubbing solution reaching the at least a predetermined threshold.
 16. A method, comprising: receiving a waste gas comprising an acid gas and at least one of an elemental halogen and a halogen-containing composition; passing the waste gas through a wet scrubber to remove the at least one of the elemental halogen and the halogen-containing composition from the waste gas and form a scrubbing solution comprising the at least one of the elemental halogen and the halogen-containing composition; sorbing at least a portion of the at least one of the elemental halogen and the halogen-containing composition on a carbonaceous material; and recovering the carbonaceous material from the scrubbing solution to form a recovered carbonaceous material.
 17. The method of claim 16, wherein the waste gas comprises elemental mercury and further comprising: contacting a halogen-containing additive with the waste gas upstream of the wet scrubber to oxidize the elemental mercury in the waste gas to a non-elemental mercury, wherein at least a portion of the removed at least one of the elemental halogen and the halogen-containing composition is contacted with the waste gas upstream of the wet scrubber.
 18. The method of claim 17, further comprising: introducing the recovered the carbonaceous material to the waste gas as the halogen-containing additive.
 19. The method of claim 16, wherein after the recovering, the scrubbing solution is returned to the wet scrubber and wherein the wet scrubber is a flue-gas desulfurization scrubber.
 20. The method of claim 16, wherein a flow of an input scrubbing solution through the wet scrubber is stopped automatically by a microprocessor when a concentration of the at least one of the elemental halogen and the halogen-containing composition in an output scrubbing solution is at least a predetermined threshold and/or a difference between concentrations of the at least one of the elemental halogen and the halogen-containing composition in the input and output scrubbing solutions is at least a predetermined threshold. 